Some automobiles have a power steering system so as to ensure a smooth operation (rotation) of the steering wheel. A power steering system gives assistance such that a pressurized oil produced in an oil pump is sent to a power cylinder via a control valve, and a steering operation is smoothed by an assist force generated in the power cylinder. A power steering system of the type controlling this assist force variably is also known widely. Common practice for controlling the assist force is to adjust the opening of the control valve, thereby adjusting the flow rate or pressure of pressure oil which is sent to the power cylinder.
Generally, a vehicle speed is used as a control parameter, but a vehicle load may be used as an alternative parameter. With such a vehicle load, vehicle speed responsive power steering system, as vehicle speed increases, or axle load (load on each wheel) decreases, it is recommendable to adjust the control valve so as to lower the flow rate or pressure of pressure oil, thereby lowering the assist force. As vehicle speed decreases, or as axle load increases, on the other hand, it is recommendable to adjust the control valve so as to raise the flow rate or pressure of pressure oil, thereby raising the assist force.
An example of a vehicle load, vehicle speed responsive power steering system may have a structure as shown in FIG. 1. As illustrated in FIG. 1, a suspension 1 of a truck comprises a leaf spring 2, a shock absorber 3, an axle (not shown), a frame 4, and a U-bolt 5.
An axle load sensor 10 takes an arm-like shape, one end thereof being rotatably mounted on the frame 4, and the other end being mounted on the axle. The axle load sensor 10 detects displacement between the frame 4 and the front axle, generating an axle load signal S.sub.1 of a value corresponding to the detected displacement.
A vehicle speed sensor 11, on the other hand, produces a vehicle speed signal S.sub.2, a pulse signal indicating a vehicle speed.
A controller 12 adjusts the opening of a control valve 13 so that an optimum steering operation can be performed in response to an axle load determined by multiplying the value of the axle load signal S.sub.1 by the spring constant of the suspension, and a vehicle speed determined by the vehicle speed signal S.sub.2 (detected vehicle speed) . The opening of the control valve 13 has been set in response to a combination of each axle load and each vehicle speed. The numeral 14 denotes a steering wheel.
Such a vehicle load, vehicle speed responsive power steering system poses the following problem when adjusting the control valve in a one-to-one relationship with a change in axle load: If the automobile is running on a rough road, or is passing over a difference in road level, the value of the detected axle load varies because of vehicle body vibrations, even when cargo load is the same. Thus, the opening of the control value 13 is sequentially adjusted, and the assist force fluctuates.
In FIG. 5, F represents the value of an axle load, and f represents an operating force for the steering wheel 14. F.sub.0 represents an axle load at the standstill of the automobile. As shown in this drawing, even when the cargo load is the same, the value of the detected axle load varies because of vehicle body vibrations. Thus, the operating force for the steering wheel 14 fluctuates while the vehicle is running. A steering feeling becomes even worse.
A conventional example paying attention to the above-described problem is present in Japanese Patent Publication No. 14672/91. The gist of this conventional example is a method for controlling a power steering system which adjusts a steering force by controlling an oil flow rate in response to a vehicle speed, the method comprising increasing an oil flow rate when an apparent average load at predetermined time intervals determined by changes in the vehicle height is large, and decreasing an oil flow rate when this average load is small.
With this conventional example disclosed in Japanese Patent Publication No. 14672/91, control is performed in response to the apparent average load at predetermined time intervals. Even in this case, the average load, consequently the assist force, varies delicately depending on the driving conditions. Thus, the aforementioned problem cannot be fully solved.
Fluctuations may be suppressed by setting a long predetermined period of time for calculating the average load. In this case, responses to changes in the cargo or the number of passengers become extremely poor.
The present invention has been accomplished in the light of the above-described problems. The invention aims at providing a vehicle load responsive power steering system having a steering feeling improved by making adjustment only when the actual cargo load varies in adjusting the steering assist force in response to changes in the vehicle load.